1. Field of Invention
The invention relates to a piezoelectric transducer and an ink ejector using a piezoelectric transducer.
2. Description of Related Art
A piezoelectric ink ejector has been conventionally proposed for a printhead. In a drop-on-demand ink ejector, a piezoelectric transducer deforms to change the volume of an ink channel containing ink. Ink in the ink channel is ejected from a nozzle when the volume is reduced, while ink is drawn into the ink channel when the volume is increased. Typically, a number of such ink ejecting mechanisms are disposed adjacent to each other, and ink is selectively ejected from an ink ejecting mechanism located in a particular position to form desired characters and graphics.
In a conventional piezoelectric ink ejector, one piezoelectric transducer is used for each ink ejecting mechanism. In this case, if a number of ink ejecting mechanisms are clustered to form an image over a wide range at high resolution, the ink ejector becomes complicated in structure and expensive to manufacture. In addition, it is hard to downsize each ejecting mechanism because the piezoelectric transducer cannot be made smaller due to machining constraints. Thus, the resolution is limited in such an ink ejector.
To address the forgoing problems, a single piezoelectric transducer disposed across a plurality of ink channels has recently been proposed for a piezoelectric ink ejector. A portion of the single piezoelectric transducer corresponding to a particular ejecting mechanism is locally deformed.
Such a piezoelectric ink ejector is disclosed in U.S. Pat. No. 6,174,051. A piezoelectric transducer of the disclosed ink ejector has, as shown in FIG. 7, an ink insulating layer 40 contacting the ink in ink channel 31, a piezoelectric ceramic layer 34 stacked on the ink insulating layer 40, and laminated piezoelectric ceramic layers 33 stacked on the piezoelectric ceramic layer 34. All of these layers 40, 34, 33 are polarized in a direction in which these layers 40, 34, 33 are laminated, as indicated by arrows E. An outer electrode 38 is provided over an upper surface of the ink insulating layer 40 and is connected to a negative terminal of a drive power source. First inner electrodes 36 are provided on an upper surface of each of the piezoelectric ceramic layers 33, 34, corresponding to each ink channel 31 and are connected to a positive terminal of the actuating power source. Second inner electrodes 37 are provided on an upper surface of each of the piezoelectric ceramic layers 33, 34, corresponding to each partition wall 35 and are connected to the negative terminal of the actuating power source.
Upon the application of a voltage to the first inner electrodes 36 corresponding to a selected ink channel 31 according to print data, an electric field is generated perpendicularly to the polarization direction, as indicated by arrows G, between the first and second inner electrodes 36, 37, and the piezoelectric ceramic layers 33 are deformed in a shear mode. At the same time, an electric field is generated in the same direction as the polarization direction, as indicated by arrows H, between the first inner electrodes 36 and the outer electrode 38, and the piezoelectric ceramic layer 34 is deformed in an extensional mode. As a result, a portion of the piezoelectric transducer corresponding to the selected ink channel 31 is deformed by a bimorph effect to increase the volume of the ink channel 31.
Thereafter, when the voltage is disconnected, the portion deformed returns to its original state to reduce the volume of the ink channel 31, thereby ejecting the ink from a nozzle (not shown).
The ink ejector structured as described above is easy and inexpensive to manufacture and able to accomplish high-resolution printing. However, in the piezoelectric transducer of the above-described ink ejector, a drive voltage is applied to the piezoelectric ceramic layers 33 in a direction different from the polarization direction repeatedly for ink ejection, and thus the polarization property tends to deteriorate. As a result, deformation of the piezoelectric transducer becomes unstable, ink ejection becomes inaccurate, and print quality is reduced.
The invention provides an improved piezoelectric transducer that can be effectively deformed upon receipt of even a low voltage without deterioration in the polarization property. The invention also provides an ink ejector that can eject a large droplet at high speed.
According to one aspect of the invention, a piezoelectric transducer includes a first piezoelectric ceramic layer and a second piezoelectric ceramic layer laminated to the first piezoelectric layer. A first electrodes set including a plurality of electrodes is provided spacedly for the first piezoelectric ceramic layer perpendicularly to a laminating direction of the first and second piezoelectric ceramic layers, and the first electrodes set defines therebetween at least a first area polarized perpendicularly to the laminating direction. A second electrodes set including a plurality of electrodes positioned in the second piezoelectric ceramic layer spacedly in the laminating direction, and the second electrodes set defining at least a second area polarized parallel to the laminating direction. At least a second area is, aligned in the laminating direction with the at least a first area.
Upon application of a voltage to the first electrodes set and to the second electrodes set, an electric field is generated in the polarization direction in each of the at least a first area and the at least a second area. As a result, at least a first area extends by a longitudinal effect perpendicularly to the laminating direction, and at least a second area contracts by a transversal effect perpendicularly to the laminating direction.
The first piezoelectric ceramic layer locally extends while the second piezoelectric ceramic layer locally contracts. The piezoelectric transducer is locally deformed effectively by such a bimorph effect produced upon the application of even a low voltage.